The present invention relates to a laser noise cancel circuit reducing laser noises arising from returned light from a disk or temperature change, and to an optical disk device using the laser noise cancel circuit.
Conventional optical disk recorder and reproducing devices have the problem that laser noises may increase due to returned light from a disk or temperature change, and may have adverse effects on reproduced signals. There has been a method of reducing such laser noises, as already disclosed in Japanese Patent Laid-open No. Hei 10-124919, that is a laser noise cancel (LNC) system for canceling laser noises by subtracting laser light noise components monitored directly from light signals modulated by the disk. With the conventional technique using the subtraction method to obtain a high depression effect against laser noises, however, in the LNC arithmetic section, the level of a RF signal, which is a reproduced signal of data recorded on an optical disk, should be brought precisely identical to the level of a front photodiode signal (hereinafter referred to as a FPD signal), which can be used as a signal for monitoring a light emitting status of the laser as well as a signal for laser noise canceling, and also the level of a FPD signal needs to be readjusted in each case because the level of a RF signal tends to vary due to individual differences between disks and dispersion within a surface.
FIG. 7 and FIG. 8 are circuit diagrams each showing configuration of a conventional laser noise cancel circuit.
First, a laser noise cancel system using the conventional subtraction method will be explained with reference to FIG. 7. While a positive phase input terminal of a differential amplifier 121 is applied with a certain bias voltage VREF, a reverse phase input terminal is connected to a cathode of a main photodiode 111 (hereinafter referred to as a main PD) with an anode thereof connected to GND, and with a terminal of a return resistor 131 of which the other terminal is connected to an output terminal of the differential amplifier 121.
Similarly, a positive phase input terminal of a differential amplifier 122 is applied with a certain bias voltage VREF, a reverse phase input terminal of the differential amplifier 122 is connected to the cathode of a front photodiode 112 (hereinafter referred to as a front PD) with the anode connected to GND, and with a terminal of a return resistor 132 with the other terminal thereof connected to an output terminal of the differential amplifying circuit 122. The output terminals of the differential amplifiers 121 and 122 are connected to inputs of the arithmetic circuit 140 to output a RF signal with reduced laser noise components (LNC signal), while the output terminal of the differential amplifying circuit 122 simultaneously outputs a FPD signal. These circuit components are composed of a single or a plurality of integrated circuit elements.
When laser light reflected by a disk is introduced into the main PD 111, the light is subjected to photoelectric conversion by the main PD 111 to become a current signal, and amplified by the differential amplifier 121 and then by the return resistor 131 to become a voltage signal to be outputted as a RF signal from the output terminal of the differential amplifier 121.
On the other hand, when light from laser is introduced into the front PD 112 monitoring a quantity of laser light, the light is subjected to photoelectric conversion by the front PD 112 to become a current signal, which is amplified and converted by the differential amplifier 122 and then by the return resistor 132 to become a voltage signal, thus providing an FPD signal corresponding on quantity of laser light at the output terminal of the differential amplifier 122.
The laser light introduced into the main PD 111 contains laser noises emitted from the laser as well as signals recorded on the disk, and similarly the light introduced into the front PD 112 from the laser contains laser noises emitted from the laser. By inputting these signals, both of which contain those laser noise components, from differential amplifier 121 and 122 into the arithmetic circuit 140 to cancel the laser noise components, a LNC signal can be extracted.
FIG. 8 is a circuit diagram showing a specific example of a laser noise cancel circuit containing a conventional arithmetic circuit 140 shown in FIG. 7. In FIG. 8, the same signs are assigned to the same or corresponding components as shown in FIG. 7 and descriptions are omitted herefrom.
In the amplifying circuit 140, one of input terminals of an adder 169 executing operation for canceling laser noises from a RF signal is connected to an output terminal of an amplifier 151 for amplifying the RF signal outputted from a differential amplifier 121, and the other input terminal is connected to an output terminal of an amplifier 152 for amplifying a FPD signal outputted from a differential amplifier 122 in reverse phase via a high pass filter (hereinafter referred to as HPF) 160 for shielding DC components. The reason for shielding DC components of the FPD signal for noise canceling by HPF 160 is to prevent the DC component of the FPD signal for noise canceling from affecting a DC component of the RF signal. In addition, since the DC voltage outputted from the HPF 160 is not determined by the HPF 160, the output terminal of the HPF 160 is connected via a resistor 168 to VREF so that a DC bias voltage is applied thereto.
By adjusting gains in the amplifier 151 and 152 according to a gain control signal provided from outside of the laser noise cancel circuit, and also by adjusting the noise levels between the RF signal and the FPD signal to be identical to each other, laser noises are canceled by the adder 169 so that a LNC signal with depressed laser noises is outputted. To cancel laser noises effectively, a cut-off frequency of the HPF 160 is to be set at a frequency which is adequately smaller than a band of the RF signal, and which can follow changes in a reflection coefficient from disk to disk and influences by decentering.
The description above assumes that the amplifier 151 is a gain-variable amplifier, but the amplifier 151 may be a gain-fixed amplifier, or may be omitted.
As the optical disk device for canceling laser noises, there is an optical disk device for canceling laser noise components contained in a read information signal obtained from a read information signal processor with a phase-inverted variable detected signal obtained from a second photo-receiver, by phase-inverting a variable detected signal obtained from the second photo-receiver and adding it to a read output signal obtained from a first photo-receiver or to the read information signal obtained from the read information signal processor is disclosed illustratively in Japanese Patent Laid-open No. Hei 10-124919.
The laser noise cancel circuit and optical disk device each based on the conventional technology as described above have the following drawbacks. Since a noise level of a RF signal, likewise RF signals, depends on a reflection coefficient of a disk or other factors and varies depending on the conditions such as non-uniformity of disks to be reproduced or irregularities on a surface of each disk, in order to constantly obtain the maximized noise canceling effects, it has to be adjusted in such a way that laser noise levels emerging in the outputs of amplifiers 151 and 152 should be identical to each other, and therefore the control is difficult and it is not easy to obtain a sufficient depression effect on laser noises.